Solid-state microcellular processing is a well-known art involving a two-stage batch process. In the first stage (absorption), a solid polymer is saturated with high pressure inert gas (e.g., CO2) in a pressure vessel until a desired gas concentration level is achieved throughout the polymer matrix. Once the gas-polymer mixture is removed from the pressure vessel into ambient environment (desorption), a supersaturated specimen is produced that is thermodynamically unstable due to the excessive diffusion of gas into the polymer. In the second stage (foaming), the gas-polymer mixture is heated in a hot water bath or some other heating medium (non-limiting examples of which include hot air, steam, infrared radiation, etc.) at a temperature close to the glass transition temperature (Tg) of the gas-polymer mixture in order to induce microcellular bubble nucleation and growth.
Saturation of polymer rolls requires placement of an interleaving layer between layers of polymer, to allow gas to enter the surface of the polymer evenly. The addition of this layer is an added process step, and the removal of the interleavement is yet another process step.
Repeated opening and closing of pressure vessels wastes large amounts of high pressure gas, and also creates pauses in plastic processing. A roll that is removed from a pressure vessel begins to lose gas immediately. Gas diffuses out into the atmosphere from the exterior of the roll first and the interior of the roll last. Hence, there may be difficulty achieving consistent foaming behavior throughout the roll, particularly in polymers that lose gas quickly.
U.S. Pat. No. 5,158,986 discloses a method of passing an extruded thermoplastic material from a gas-saturation chamber to a foaming chamber via a time-controlled system of rollers. Dynamic seals are stationed between the chambers to allow passage of the thermoplastic sheet while preserving the environmental conditions of each chamber. To date, however, there is no engineering solution for manufacturing a dynamic seal that can operate at pressures required for practicing the art referenced in this current patent.